Effects of Water Stress under Contrasting Environmental Conditions on Transpiration and Photosynthesis in Soybean

Continuous and simultaneous measurements of CO₂ exchange andtranspiration rates of whole soybean plants were made undercontrasting, controlled environmental conditions when waterstress was imposed by withholding water. Daytime temperaturesand vapour pressure deficits were 27.5 ° C/12 mbar; 27.5° C/5 mbar; 22.5 ° C/12 mbar, and 22.5 ° C/5 mbar.The experimental conditions were virtually the same as the conditionsunder which the plants had been grown. Under all four treatments photosynthesis and transpiration rateswere closely correlated as water stress increased, but in viewof the evidence for a significant mesophyll resistance to photosynthesisin- both stressed and unstressed plants it is not consideredthat this is due to total stomatal control. At — 0.4 bar soil water potential (_soll) the rates oftranspiration and photosynthesis became independent of the atmosphericconditions and were very similar under all treatments. Thisis attributed to slow movement of water into the root zone fromthe surrounding soil with associated stomatal closure limitingthe rates of water uptake and transpiration. With decreasing _soll, relative water content of the leaf (RWC)fell more rapidly and to lower levels under 27.5 ° C/12mbar conditions than under the other treatments. The least reductionin RWC was under the 22.5 ° C/5 mbar treatment. Increasingsoil water stress had the greatest relative effect on ratesof gaseous exchange under 27.5 ° C/12mbar conditions andleast under 22.5 ° C/5mbar conditions.     

Transpiration and Photosynthesis in Soybean: EFFECTS OF TEMPERATURE AND VAPOUR PRESSURE DEFICIT

Continuous and simultaneous measurements of CO₂ exchange andtranspiration rates of whole soybean plants were made undercontrasting, controlled environmental conditions for periodsof up to 3 d. Daytime temperatures and vapour pressure deficits(VPD) were 27.5 °C/12 mb; 27.5 °C/5 mb; 22.5 °C/12mb, and 22.5 °C/5 mb. Night temperatures were 5°C lowerthan day temperatures and night VPD was 2.7 mb and 3.5 mb atthe higher and lower temperature respectively. The experimentalconditions were virtually the same as those under which theplants had been grown. Transpiration rates were higher at the higher VPD but were alsoinfluenced by temperature. At 12 mb VPD the rates were 16 percent lower at 22.5 °C than at 27.5 °C. Temperature hadno effect on the transpiration rate at 5 mb VPD. Photosynthesis rates were lower at 5 mb VPD than at 12 mb VPDat both temperatures: the difference was substantially greater(c. 70 per cent) at 22.5 °C. Under all treatments meso-phyllresistance (r'_m) appeared to have a major effect on the photosyntheticrate, and varied more than twofold between treatments. r'_m washighest in plants grown at 22.5 °C/5 mb VPD and lowest at27.5 °C/12 mb VPD.